Filters, in particular high-frequency filters, are used in a wide variety of electronic devices for many different applications and functions. One common implementation of a high-frequency filter that is used in wireless communications devices, for example, in the antenna duplexer of such devices, is a ladder-type filter such as that shown in FIG. 1. Referring to FIG. 1, the filter 100 includes input/output terminals 102 and 104, a signal line 106, one or more series arm resonators 108, and a plurality of parallel arm resonators 110. The signal line 106 is connected between the input/output terminals 102 and 104, as shown. The series arm resonator(s) 108 are connected in series with each other along the signal line 106. The parallel arm resonator(s) 110 are connected between the signal line 106 and a reference potential 112. The reference potential 112 can be ground. Together, the series arm resonators 108 and parallel arm resonators 110 form a ladder circuit and function as a band-pass filter having a passband and stopbands. In certain examples, a phase shifter 114, such as an inductor, can be included (as shown in FIG. 1) as a matching component, particularly when the filter 100 is used in an antenna duplexer.
In certain modern communications devices, such as mobile phones, for example, there is an ongoing trend to make these devices smaller, and accordingly, it can be desirable to reduce the number of components used in the communications “front end” portion (e.g., the radio frequency portion) of such devices. In the radio frequency circuit portion of certain wireless communications devices, the number of components can be reduced by reducing the number of inter-stage filters in the signal transmission and reception paths. However, this requires the antenna duplexer to have a very good isolation characteristic. As a result, it can be necessary for bandpass filters used in the antenna duplexer to have very high attenuation in the stopband(s). These design requirements can be challenging and are not met by conventional ladder-type filters.
Certain antenna duplexer designs based on ladder-type filters, such as that shown in FIG. 1, incorporate the use of a phase-influencing circuit to improve the isolation characteristic of the duplexer. For example, U.S. Pat. No. 9,246,533 discloses various examples and embodiments of electronic devices, including antenna duplexers, which incorporate an auxiliary circuit connected in parallel with a main circuit that includes one or more filters, the auxiliary circuit having a phase characteristic opposite to that of the main circuit at certain frequencies. U.S. Pat. No. 9,246,533 discloses several variations of the auxiliary circuit, which can include one or more surface acoustic wave (SAW) resonators that are formed on a piezoelectric substrate. As disclosed in U.S. Pat. No. 9,246,533, the main circuit can have a certain frequency response, also referred to as a passing characteristic, that corresponds to the passbands and stopbands of the filters included therein. The auxiliary circuit can be designed such that, within a certain frequency band, it has passing characteristic with an amplitude that is substantially similar to the amplitude of the passing characteristic of the main circuit, and a phase that opposite to the phase of the passing characteristic of the main circuit. As a result, in the certain frequency band, a main signal output from the main circuit is at least partially canceled by an auxiliary signal output from the auxiliary circuit, thereby improving the isolation characteristic of the antenna duplexer.